1. Field of the Invention:
This invention relates to current limiting fuses and, more particularly, to fuses as having indicator means for denoting a fuse condition.
2. Description of the Prior Art:
One type of current limiting fuse comprises a cylindrical housing of insulating material having opposite ends closed by a metal cap or ferrule. At least one fusible element extends through the housing and between the caps. During normal current conditions, the fusible elements remain in tact. When current through the fuse element exceeds the current rating of the fuse for a sufficient time, the element melts and usually forms an arc, for which reason the housing is filled with an arc-extinguishing material such as sand.
Aside from the fact that a fuse no longer functions after the fuse elements melt, there is normally no obvious change in the outward appearance of the fuse. For that reason, fuses that are currently in use are provided with an indicator by which it is readily evident that the fuse has melted, or fused. Some examples of fuses having indicators are disclosed in U.S. Pat. Nos. 3,636,491, 3,848,445, 3,911,385, 4,058,784, and 4,060,786. The indicators are normally retained in a retracted position by a restraining wire which, like the fuse elements, extends throughout the length of the casing and is in electrical contact with the terminals at opposite ends thereof.
Under normal operating conditions of the fuse, the indicator wire does not conduct a significant current because it has a higher coefficient of electrical resistance than the fuse elements. Only after the fuse elements have melted, or fused, does the restraining wire conduct current momentarily until it likewise melts or fuses, at which time the indicator is no longer retracted out of position and is extended to an indicating position. With some fuses, however, the gauge of the restraining wire must be sufficiently large for fabrication and connection with an indicator, and at the same time small enough to melt in a minimal required time.
In addition, it is necessary during assembly of the indicator that electrical contact is maintained between the several parts to assure a closed circuit through the restraining wire when the wire is bearing the load after the fuse elements have melted.